The present invention relates to the field of photography and, more specifically, to automatic exposure control systems for cameras which utilize a dynamic aperture scanning shutter of the type wherein an integrated blade mechanism performs both the effective aperture and exposure interval defining functions.
The integrated blade mechanism may be of the type including a pair of elongated blade members mounted in superposed relation for simultaneous movement, in opposite directions, between a closed position and a maximum aperture defining open position. Generally, the blades have complementary longitudinally tapered aperture defining openings therein arranged with their respective narrower ends in facing relation.
When the blades are in the closed position, their openings are completely out of registration and the blades block transmission of image forming light from a scene to the camera film plane. To initiate exposure, the blades are moved simultaneously in opposite directions causing the openings to progressively overlap and define a light transmission aperture that increases in size as a function of blade displacement from the closed position. Upon reversing the direction of blade drive, the light transmission aperture progressively decreases in size until it closes to terminate the exposure interval.
Other types of scanning shutters employ multiple pivoting blades arranged in an iris diaphragm configuration but basically operate in the same manner as the two-bladed shutter.
The characteristics of a given ambient light exposure made with a scanning shutter can be portrayed graphically by plotting an aperture size (area) versus time trajectory curve. The area bounded by the trajectory is indicative of the total quantity of light that reaches the film to effect exposure. The shape of the trajectory, however, indicates the nature of the exposure in terms of depth of field, motion stopping ability and other exposure parameters.
For example, assuming adequate ambient light conditions for a particular type of film, one trajectory that bounds a given area A may be generally triangular in shape and show that the blades open slowly to a relatively small peak aperture and then close at the same rate. Thus the slope of the opening and closing segments are fairly shallow. In this case the average or effective aperture is small compared to the maximum obtainable aperture and the recorded image will exhibit sharp focus over a relatively deep field.
A second triangular trajectory that bounds the same given area A (thereby providing the same exposure level as the first exposure) may indicate that the blades open quickly to a relatively large peak aperture and then close quickly at the same rate. The slope of the opening and closing segments of the trajectory will be fairly steep but the exposure interval will be shorter than in the first exposure. In this instance, the short exposure interval provides good motion stopping ability. However, because the effective aperture is relatively large, the depth of field of the recorded image will not be as deep as it was in the first exposure.
In addition to dependability and cost effectiveness, a very important characteristic and design goal of any exposure control system incorporating a scanning shutter is versatility.
In terms of photographic performance, versatility can be rated by the system's ability, or lack thereof, to accurately generate a wide variety of different shaped trajectory curves, both automatically and in response to user inputs, to accommodate varying scene conditions, different film speeds and/or exposure characteristics, and also to modify the exposure characteristics for a given set of scene conditions.
Another measure of a system's versatility relates to its potential for wide use and sustained longevity in a climate of rapidly changing technology. That is, can it be economically incorporated into a camera product line that varies in sophistication and price? And, perhaps more importantly, is it capable of being updated periodically with relatively inexpensive modifications to take advantage of newly developing exposure ideas in areas such as optimization of trajectory curves for the best compromise between motion stopping ability and depth of field; or development of trajectory curves that enhance the performance of a particular optical system.
Scanning shutter systems wherein the rate of blade displacement can be varied to generate different trajectories are, of course, known in the prior art. For example, U.S. Pat. Nos. 3,733,991 and 3,820,131 disclose shutters wherein the rate at which the spring driven blades are displaced is controlled by either a mechanical or electromagnetic braking system. In the first patent, the braking action is varied in response to aperture and exposure interval inputs provided when the user manually sets corresponding input rings on the shutter housing. As shown in FIG. 16, it is suggested that the braking action may be varied during the exposure interval to effect blade acceleration and deceleration thereby generating trajectory curves which have non-linear portions. In the second patent, the braking action is set automatically in accordance with the scene brightness levels monitored by a photocell circuit. While these systems provide some performance versatility, their mechanical complexity limits their ability to be used in a number of different camera models and to be easily updated with only minimum modification.
Therefore, it is an object of the present invention to provide an exposure control system for use in photographic apparatus that is highly versatile in both its photographic performance and ability to be economically incorporated into a number of different camera systems as well as being able to be easily updated with minimum modification.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.
The exposure control system of the present invention utilizes an integrated blade mechanism driven by a stepper motor under the control of a microcomputer which is programmed to draw upon stored trajectory data, in response to selected inputs, and develop an appropriate set or program of motor driving trajectory signals to provide a corresponding trajectory curve. Such a system allows the shutter to be programmed for the generation of as many different trajectories as is appropriate for the level of sophistication of the camera in which it is to be used. Also, the system may be updated periodically for use in new products simply by reprogramming.
Certain elements of the exposure control system embodying the present invention are known in the prior art. For example, commonly-assigned U.S. Pat. Nos. 3,900,855 and 3,945,025 show a scanning shutter driven by a stepper motor. In U.S. Pat. No. 4,053,907 an elongated array of light chopping slits are cut through one of the shutter blades so that blade displacement may be optically detected in order to generate pulse signals which control the stepwise rotation of the stepper motor. U.S. Pat. Nos. 3,628,119 and 3,636,429 are relevant for showing closed loop stepper motor control systems. U.S. Pat. No. 4,066,347 is relevant for showing a camera wherein the frequency of stepper motor drive pulses is varied to change the rate at which aperture defining blades are displaced. A disclosure by D. M. Harvey appearing in Research Disclosure, February 1980, No. 19041, entitled "Microprocessor Exposure Control In A Camera" is relevant for showing an exposure control system that is operated by a microprocessor and is responsive to automatic and manual inputs for selecting a proper one of a plurality of exposure control programs from a storage unit.